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 * Copyright (c) 1999, 2008, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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package java.util;

import java.util.Date;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * A facility for threads to schedule tasks for future execution in a
 * background thread.  Tasks may be scheduled for one-time execution, or for
 * repeated execution at regular intervals.
 *
 * <p>Corresponding to each <tt>Timer</tt> object is a single background
 * thread that is used to execute all of the timer's tasks, sequentially.
 * Timer tasks should complete quickly.  If a timer task takes excessive time
 * to complete, it "hogs" the timer's task execution thread.  This can, in
 * turn, delay the execution of subsequent tasks, which may "bunch up" and
 * execute in rapid succession when (and if) the offending task finally
 * completes.
 *
 * <p>After the last live reference to a <tt>Timer</tt> object goes away
 * <i>and</i> all outstanding tasks have completed execution, the timer's task
 * execution thread terminates gracefully (and becomes subject to garbage
 * collection).  However, this can take arbitrarily long to occur.  By
 * default, the task execution thread does not run as a <i>daemon thread</i>,
 * so it is capable of keeping an application from terminating.  If a caller
 * wants to terminate a timer's task execution thread rapidly, the caller
 * should invoke the timer's <tt>cancel</tt> method.
 *
 * <p>If the timer's task execution thread terminates unexpectedly, for
 * example, because its <tt>stop</tt> method is invoked, any further
 * attempt to schedule a task on the timer will result in an
 * <tt>IllegalStateException</tt>, as if the timer's <tt>cancel</tt>
 * method had been invoked.
 *
 * <p>This class is thread-safe: multiple threads can share a single
 * <tt>Timer</tt> object without the need for external synchronization.
 *
 * <p>This class does <i>not</i> offer real-time guarantees: it schedules
 * tasks using the <tt>Object.wait(long)</tt> method.
 *
 * <p>Java 5.0 introduced the {@code java.util.concurrent} package and
 * one of the concurrency utilities therein is the {@link
 * java.util.concurrent.ScheduledThreadPoolExecutor
 * ScheduledThreadPoolExecutor} which is a thread pool for repeatedly
 * executing tasks at a given rate or delay.  It is effectively a more
 * versatile replacement for the {@code Timer}/{@code TimerTask}
 * combination, as it allows multiple service threads, accepts various
 * time units, and doesn't require subclassing {@code TimerTask} (just
 * implement {@code Runnable}).  Configuring {@code
 * ScheduledThreadPoolExecutor} with one thread makes it equivalent to
 * {@code Timer}.
 *
 * <p>Implementation note: This class scales to large numbers of concurrently
 * scheduled tasks (thousands should present no problem).  Internally,
 * it uses a binary heap to represent its task queue, so the cost to schedule
 * a task is O(log n), where n is the number of concurrently scheduled tasks.
 *
 * <p>Implementation note: All constructors start a timer thread.
 *
 * @author Josh Bloch
 * @see TimerTask
 * @see Object#wait(long)
 * @since 1.3
 */

public class Timer {

  /**
   * The timer task queue.  This data structure is shared with the timer
   * thread.  The timer produces tasks, via its various schedule calls,
   * and the timer thread consumes, executing timer tasks as appropriate,
   * and removing them from the queue when they're obsolete.
   */
  private final TaskQueue queue = new TaskQueue();

  /**
   * The timer thread.
   */
  private final TimerThread thread = new TimerThread(queue);

  /**
   * This object causes the timer's task execution thread to exit
   * gracefully when there are no live references to the Timer object and no
   * tasks in the timer queue.  It is used in preference to a finalizer on
   * Timer as such a finalizer would be susceptible to a subclass's
   * finalizer forgetting to call it.
   */
  private final Object threadReaper = new Object() {
    protected void finalize() throws Throwable {
      synchronized (queue) {
        thread.newTasksMayBeScheduled = false;
        queue.notify(); // In case queue is empty.
      }
    }
  };

  /**
   * This ID is used to generate thread names.
   */
  private final static AtomicInteger nextSerialNumber = new AtomicInteger(0);

  private static int serialNumber() {
    return nextSerialNumber.getAndIncrement();
  }

  /**
   * Creates a new timer.  The associated thread does <i>not</i>
   * {@linkplain Thread#setDaemon run as a daemon}.
   */
  public Timer() {
    this("Timer-" + serialNumber());
  }

  /**
   * Creates a new timer whose associated thread may be specified to
   * {@linkplain Thread#setDaemon run as a daemon}.
   * A daemon thread is called for if the timer will be used to
   * schedule repeating "maintenance activities", which must be
   * performed as long as the application is running, but should not
   * prolong the lifetime of the application.
   *
   * @param isDaemon true if the associated thread should run as a daemon.
   */
  public Timer(boolean isDaemon) {
    this("Timer-" + serialNumber(), isDaemon);
  }

  /**
   * Creates a new timer whose associated thread has the specified name.
   * The associated thread does <i>not</i>
   * {@linkplain Thread#setDaemon run as a daemon}.
   *
   * @param name the name of the associated thread
   * @throws NullPointerException if {@code name} is null
   * @since 1.5
   */
  public Timer(String name) {
    thread.setName(name);
    thread.start();
  }

  /**
   * Creates a new timer whose associated thread has the specified name,
   * and may be specified to
   * {@linkplain Thread#setDaemon run as a daemon}.
   *
   * @param name the name of the associated thread
   * @param isDaemon true if the associated thread should run as a daemon
   * @throws NullPointerException if {@code name} is null
   * @since 1.5
   */
  public Timer(String name, boolean isDaemon) {
    thread.setName(name);
    thread.setDaemon(isDaemon);
    thread.start();
  }

  /**
   * Schedules the specified task for execution after the specified delay.
   *
   * @param task task to be scheduled.
   * @param delay delay in milliseconds before task is to be executed.
   * @throws IllegalArgumentException if <tt>delay</tt> is negative, or <tt>delay +
   * System.currentTimeMillis()</tt> is negative.
   * @throws IllegalStateException if task was already scheduled or cancelled, timer was cancelled,
   * or timer thread terminated.
   * @throws NullPointerException if {@code task} is null
   */
  public void schedule(TimerTask task, long delay) {
    if (delay < 0) {
      throw new IllegalArgumentException("Negative delay.");
    }
    sched(task, System.currentTimeMillis() + delay, 0);
  }

  /**
   * Schedules the specified task for execution at the specified time.  If
   * the time is in the past, the task is scheduled for immediate execution.
   *
   * @param task task to be scheduled.
   * @param time time at which task is to be executed.
   * @throws IllegalArgumentException if <tt>time.getTime()</tt> is negative.
   * @throws IllegalStateException if task was already scheduled or cancelled, timer was cancelled,
   * or timer thread terminated.
   * @throws NullPointerException if {@code task} or {@code time} is null
   */
  public void schedule(TimerTask task, Date time) {
    sched(task, time.getTime(), 0);
  }

  /**
   * Schedules the specified task for repeated <i>fixed-delay execution</i>,
   * beginning after the specified delay.  Subsequent executions take place
   * at approximately regular intervals separated by the specified period.
   *
   * <p>In fixed-delay execution, each execution is scheduled relative to
   * the actual execution time of the previous execution.  If an execution
   * is delayed for any reason (such as garbage collection or other
   * background activity), subsequent executions will be delayed as well.
   * In the long run, the frequency of execution will generally be slightly
   * lower than the reciprocal of the specified period (assuming the system
   * clock underlying <tt>Object.wait(long)</tt> is accurate).
   *
   * <p>Fixed-delay execution is appropriate for recurring activities
   * that require "smoothness."  In other words, it is appropriate for
   * activities where it is more important to keep the frequency accurate
   * in the short run than in the long run.  This includes most animation
   * tasks, such as blinking a cursor at regular intervals.  It also includes
   * tasks wherein regular activity is performed in response to human
   * input, such as automatically repeating a character as long as a key
   * is held down.
   *
   * @param task task to be scheduled.
   * @param delay delay in milliseconds before task is to be executed.
   * @param period time in milliseconds between successive task executions.
   * @throws IllegalArgumentException if {@code delay < 0}, or {@code delay +
   * System.currentTimeMillis() < 0}, or {@code period <= 0}
   * @throws IllegalStateException if task was already scheduled or cancelled, timer was cancelled,
   * or timer thread terminated.
   * @throws NullPointerException if {@code task} is null
   */
  public void schedule(TimerTask task, long delay, long period) {
    if (delay < 0) {
      throw new IllegalArgumentException("Negative delay.");
    }
    if (period <= 0) {
      throw new IllegalArgumentException("Non-positive period.");
    }
    sched(task, System.currentTimeMillis() + delay, -period);
  }

  /**
   * Schedules the specified task for repeated <i>fixed-delay execution</i>,
   * beginning at the specified time. Subsequent executions take place at
   * approximately regular intervals, separated by the specified period.
   *
   * <p>In fixed-delay execution, each execution is scheduled relative to
   * the actual execution time of the previous execution.  If an execution
   * is delayed for any reason (such as garbage collection or other
   * background activity), subsequent executions will be delayed as well.
   * In the long run, the frequency of execution will generally be slightly
   * lower than the reciprocal of the specified period (assuming the system
   * clock underlying <tt>Object.wait(long)</tt> is accurate).  As a
   * consequence of the above, if the scheduled first time is in the past,
   * it is scheduled for immediate execution.
   *
   * <p>Fixed-delay execution is appropriate for recurring activities
   * that require "smoothness."  In other words, it is appropriate for
   * activities where it is more important to keep the frequency accurate
   * in the short run than in the long run.  This includes most animation
   * tasks, such as blinking a cursor at regular intervals.  It also includes
   * tasks wherein regular activity is performed in response to human
   * input, such as automatically repeating a character as long as a key
   * is held down.
   *
   * @param task task to be scheduled.
   * @param firstTime First time at which task is to be executed.
   * @param period time in milliseconds between successive task executions.
   * @throws IllegalArgumentException if {@code firstTime.getTime() < 0}, or {@code period <= 0}
   * @throws IllegalStateException if task was already scheduled or cancelled, timer was cancelled,
   * or timer thread terminated.
   * @throws NullPointerException if {@code task} or {@code firstTime} is null
   */
  public void schedule(TimerTask task, Date firstTime, long period) {
    if (period <= 0) {
      throw new IllegalArgumentException("Non-positive period.");
    }
    sched(task, firstTime.getTime(), -period);
  }

  /**
   * Schedules the specified task for repeated <i>fixed-rate execution</i>,
   * beginning after the specified delay.  Subsequent executions take place
   * at approximately regular intervals, separated by the specified period.
   *
   * <p>In fixed-rate execution, each execution is scheduled relative to the
   * scheduled execution time of the initial execution.  If an execution is
   * delayed for any reason (such as garbage collection or other background
   * activity), two or more executions will occur in rapid succession to
   * "catch up."  In the long run, the frequency of execution will be
   * exactly the reciprocal of the specified period (assuming the system
   * clock underlying <tt>Object.wait(long)</tt> is accurate).
   *
   * <p>Fixed-rate execution is appropriate for recurring activities that
   * are sensitive to <i>absolute</i> time, such as ringing a chime every
   * hour on the hour, or running scheduled maintenance every day at a
   * particular time.  It is also appropriate for recurring activities
   * where the total time to perform a fixed number of executions is
   * important, such as a countdown timer that ticks once every second for
   * ten seconds.  Finally, fixed-rate execution is appropriate for
   * scheduling multiple repeating timer tasks that must remain synchronized
   * with respect to one another.
   *
   * @param task task to be scheduled.
   * @param delay delay in milliseconds before task is to be executed.
   * @param period time in milliseconds between successive task executions.
   * @throws IllegalArgumentException if {@code delay < 0}, or {@code delay +
   * System.currentTimeMillis() < 0}, or {@code period <= 0}
   * @throws IllegalStateException if task was already scheduled or cancelled, timer was cancelled,
   * or timer thread terminated.
   * @throws NullPointerException if {@code task} is null
   */
  public void scheduleAtFixedRate(TimerTask task, long delay, long period) {
    if (delay < 0) {
      throw new IllegalArgumentException("Negative delay.");
    }
    if (period <= 0) {
      throw new IllegalArgumentException("Non-positive period.");
    }
    sched(task, System.currentTimeMillis() + delay, period);
  }

  /**
   * Schedules the specified task for repeated <i>fixed-rate execution</i>,
   * beginning at the specified time. Subsequent executions take place at
   * approximately regular intervals, separated by the specified period.
   *
   * <p>In fixed-rate execution, each execution is scheduled relative to the
   * scheduled execution time of the initial execution.  If an execution is
   * delayed for any reason (such as garbage collection or other background
   * activity), two or more executions will occur in rapid succession to
   * "catch up."  In the long run, the frequency of execution will be
   * exactly the reciprocal of the specified period (assuming the system
   * clock underlying <tt>Object.wait(long)</tt> is accurate).  As a
   * consequence of the above, if the scheduled first time is in the past,
   * then any "missed" executions will be scheduled for immediate "catch up"
   * execution.
   *
   * <p>Fixed-rate execution is appropriate for recurring activities that
   * are sensitive to <i>absolute</i> time, such as ringing a chime every
   * hour on the hour, or running scheduled maintenance every day at a
   * particular time.  It is also appropriate for recurring activities
   * where the total time to perform a fixed number of executions is
   * important, such as a countdown timer that ticks once every second for
   * ten seconds.  Finally, fixed-rate execution is appropriate for
   * scheduling multiple repeating timer tasks that must remain synchronized
   * with respect to one another.
   *
   * @param task task to be scheduled.
   * @param firstTime First time at which task is to be executed.
   * @param period time in milliseconds between successive task executions.
   * @throws IllegalArgumentException if {@code firstTime.getTime() < 0} or {@code period <= 0}
   * @throws IllegalStateException if task was already scheduled or cancelled, timer was cancelled,
   * or timer thread terminated.
   * @throws NullPointerException if {@code task} or {@code firstTime} is null
   */
  public void scheduleAtFixedRate(TimerTask task, Date firstTime,
      long period) {
    if (period <= 0) {
      throw new IllegalArgumentException("Non-positive period.");
    }
    sched(task, firstTime.getTime(), period);
  }

  /**
   * Schedule the specified timer task for execution at the specified
   * time with the specified period, in milliseconds.  If period is
   * positive, the task is scheduled for repeated execution; if period is
   * zero, the task is scheduled for one-time execution. Time is specified
   * in Date.getTime() format.  This method checks timer state, task state,
   * and initial execution time, but not period.
   *
   * @throws IllegalArgumentException if <tt>time</tt> is negative.
   * @throws IllegalStateException if task was already scheduled or cancelled, timer was cancelled,
   * or timer thread terminated.
   * @throws NullPointerException if {@code task} is null
   */
  private void sched(TimerTask task, long time, long period) {
    if (time < 0) {
      throw new IllegalArgumentException("Illegal execution time.");
    }

    // Constrain value of period sufficiently to prevent numeric
    // overflow while still being effectively infinitely large.
    if (Math.abs(period) > (Long.MAX_VALUE >> 1)) {
      period >>= 1;
    }

    synchronized (queue) {
      if (!thread.newTasksMayBeScheduled) {
        throw new IllegalStateException("Timer already cancelled.");
      }

      synchronized (task.lock) {
        if (task.state != TimerTask.VIRGIN) {
          throw new IllegalStateException(
              "Task already scheduled or cancelled");
        }
        task.nextExecutionTime = time;
        task.period = period;
        task.state = TimerTask.SCHEDULED;
      }

      queue.add(task);
      if (queue.getMin() == task) {
        queue.notify();
      }
    }
  }

  /**
   * Terminates this timer, discarding any currently scheduled tasks.
   * Does not interfere with a currently executing task (if it exists).
   * Once a timer has been terminated, its execution thread terminates
   * gracefully, and no more tasks may be scheduled on it.
   *
   * <p>Note that calling this method from within the run method of a
   * timer task that was invoked by this timer absolutely guarantees that
   * the ongoing task execution is the last task execution that will ever
   * be performed by this timer.
   *
   * <p>This method may be called repeatedly; the second and subsequent
   * calls have no effect.
   */
  public void cancel() {
    synchronized (queue) {
      thread.newTasksMayBeScheduled = false;
      queue.clear();
      queue.notify();  // In case queue was already empty.
    }
  }

  /**
   * Removes all cancelled tasks from this timer's task queue.  <i>Calling
   * this method has no effect on the behavior of the timer</i>, but
   * eliminates the references to the cancelled tasks from the queue.
   * If there are no external references to these tasks, they become
   * eligible for garbage collection.
   *
   * <p>Most programs will have no need to call this method.
   * It is designed for use by the rare application that cancels a large
   * number of tasks.  Calling this method trades time for space: the
   * runtime of the method may be proportional to n + c log n, where n
   * is the number of tasks in the queue and c is the number of cancelled
   * tasks.
   *
   * <p>Note that it is permissible to call this method from within a
   * a task scheduled on this timer.
   *
   * @return the number of tasks removed from the queue.
   * @since 1.5
   */
  public int purge() {
    int result = 0;

    synchronized (queue) {
      for (int i = queue.size(); i > 0; i--) {
        if (queue.get(i).state == TimerTask.CANCELLED) {
          queue.quickRemove(i);
          result++;
        }
      }

      if (result != 0) {
        queue.heapify();
      }
    }

    return result;
  }
}

/**
 * This "helper class" implements the timer's task execution thread, which
 * waits for tasks on the timer queue, executions them when they fire,
 * reschedules repeating tasks, and removes cancelled tasks and spent
 * non-repeating tasks from the queue.
 */
class TimerThread extends Thread {

  /**
   * This flag is set to false by the reaper to inform us that there
   * are no more live references to our Timer object.  Once this flag
   * is true and there are no more tasks in our queue, there is no
   * work left for us to do, so we terminate gracefully.  Note that
   * this field is protected by queue's monitor!
   */
  boolean newTasksMayBeScheduled = true;

  /**
   * Our Timer's queue.  We store this reference in preference to
   * a reference to the Timer so the reference graph remains acyclic.
   * Otherwise, the Timer would never be garbage-collected and this
   * thread would never go away.
   */
  private TaskQueue queue;

  TimerThread(TaskQueue queue) {
    this.queue = queue;
  }

  public void run() {
    try {
      mainLoop();
    } finally {
      // Someone killed this Thread, behave as if Timer cancelled
      synchronized (queue) {
        newTasksMayBeScheduled = false;
        queue.clear();  // Eliminate obsolete references
      }
    }
  }

  /**
   * The main timer loop.  (See class comment.)
   */
  private void mainLoop() {
    while (true) {
      try {
        TimerTask task;
        boolean taskFired;
        synchronized (queue) {
          // Wait for queue to become non-empty
          while (queue.isEmpty() && newTasksMayBeScheduled) {
            queue.wait();
          }
          if (queue.isEmpty()) {
            break; // Queue is empty and will forever remain; die
          }

          // Queue nonempty; look at first evt and do the right thing
          long currentTime, executionTime;
          task = queue.getMin();
          synchronized (task.lock) {
            if (task.state == TimerTask.CANCELLED) {
              queue.removeMin();
              continue;  // No action required, poll queue again
            }
            currentTime = System.currentTimeMillis();
            executionTime = task.nextExecutionTime;
            if (taskFired = (executionTime <= currentTime)) {
              if (task.period == 0) { // Non-repeating, remove
                queue.removeMin();
                task.state = TimerTask.EXECUTED;
              } else { // Repeating task, reschedule
                queue.rescheduleMin(
                    task.period < 0 ? currentTime - task.period
                        : executionTime + task.period);
              }
            }
          }
          if (!taskFired) // Task hasn't yet fired; wait
          {
            queue.wait(executionTime - currentTime);
          }
        }
        if (taskFired)  // Task fired; run it, holding no locks
        {
          task.run();
        }
      } catch (InterruptedException e) {
      }
    }
  }
}

/**
 * This class represents a timer task queue: a priority queue of TimerTasks,
 * ordered on nextExecutionTime.  Each Timer object has one of these, which it
 * shares with its TimerThread.  Internally this class uses a heap, which
 * offers log(n) performance for the add, removeMin and rescheduleMin
 * operations, and constant time performance for the getMin operation.
 */
class TaskQueue {

  /**
   * Priority queue represented as a balanced binary heap: the two children
   * of queue[n] are queue[2*n] and queue[2*n+1].  The priority queue is
   * ordered on the nextExecutionTime field: The TimerTask with the lowest
   * nextExecutionTime is in queue[1] (assuming the queue is nonempty).  For
   * each node n in the heap, and each descendant of n, d,
   * n.nextExecutionTime <= d.nextExecutionTime.
   */
  private TimerTask[] queue = new TimerTask[128];

  /**
   * The number of tasks in the priority queue.  (The tasks are stored in
   * queue[1] up to queue[size]).
   */
  private int size = 0;

  /**
   * Returns the number of tasks currently on the queue.
   */
  int size() {
    return size;
  }

  /**
   * Adds a new task to the priority queue.
   */
  void add(TimerTask task) {
    // Grow backing store if necessary
    if (size + 1 == queue.length) {
      queue = Arrays.copyOf(queue, 2 * queue.length);
    }

    queue[++size] = task;
    fixUp(size);
  }

  /**
   * Return the "head task" of the priority queue.  (The head task is an
   * task with the lowest nextExecutionTime.)
   */
  TimerTask getMin() {
    return queue[1];
  }

  /**
   * Return the ith task in the priority queue, where i ranges from 1 (the
   * head task, which is returned by getMin) to the number of tasks on the
   * queue, inclusive.
   */
  TimerTask get(int i) {
    return queue[i];
  }

  /**
   * Remove the head task from the priority queue.
   */
  void removeMin() {
    queue[1] = queue[size];
    queue[size--] = null;  // Drop extra reference to prevent memory leak
    fixDown(1);
  }

  /**
   * Removes the ith element from queue without regard for maintaining
   * the heap invariant.  Recall that queue is one-based, so
   * 1 <= i <= size.
   */
  void quickRemove(int i) {
    assert i <= size;

    queue[i] = queue[size];
    queue[size--] = null;  // Drop extra ref to prevent memory leak
  }

  /**
   * Sets the nextExecutionTime associated with the head task to the
   * specified value, and adjusts priority queue accordingly.
   */
  void rescheduleMin(long newTime) {
    queue[1].nextExecutionTime = newTime;
    fixDown(1);
  }

  /**
   * Returns true if the priority queue contains no elements.
   */
  boolean isEmpty() {
    return size == 0;
  }

  /**
   * Removes all elements from the priority queue.
   */
  void clear() {
    // Null out task references to prevent memory leak
    for (int i = 1; i <= size; i++) {
      queue[i] = null;
    }

    size = 0;
  }

  /**
   * Establishes the heap invariant (described above) assuming the heap
   * satisfies the invariant except possibly for the leaf-node indexed by k
   * (which may have a nextExecutionTime less than its parent's).
   *
   * This method functions by "promoting" queue[k] up the hierarchy
   * (by swapping it with its parent) repeatedly until queue[k]'s
   * nextExecutionTime is greater than or equal to that of its parent.
   */
  private void fixUp(int k) {
    while (k > 1) {
      int j = k >> 1;
      if (queue[j].nextExecutionTime <= queue[k].nextExecutionTime) {
        break;
      }
      TimerTask tmp = queue[j];
      queue[j] = queue[k];
      queue[k] = tmp;
      k = j;
    }
  }

  /**
   * Establishes the heap invariant (described above) in the subtree
   * rooted at k, which is assumed to satisfy the heap invariant except
   * possibly for node k itself (which may have a nextExecutionTime greater
   * than its children's).
   *
   * This method functions by "demoting" queue[k] down the hierarchy
   * (by swapping it with its smaller child) repeatedly until queue[k]'s
   * nextExecutionTime is less than or equal to those of its children.
   */
  private void fixDown(int k) {
    int j;
    while ((j = k << 1) <= size && j > 0) {
      if (j < size &&
          queue[j].nextExecutionTime > queue[j + 1].nextExecutionTime) {
        j++; // j indexes smallest kid
      }
      if (queue[k].nextExecutionTime <= queue[j].nextExecutionTime) {
        break;
      }
      TimerTask tmp = queue[j];
      queue[j] = queue[k];
      queue[k] = tmp;
      k = j;
    }
  }

  /**
   * Establishes the heap invariant (described above) in the entire tree,
   * assuming nothing about the order of the elements prior to the call.
   */
  void heapify() {
    for (int i = size / 2; i >= 1; i--) {
      fixDown(i);
    }
  }
}
